1. Field of the Invention
The invention pertains to the field of digital communications and, more particularly, to the symbol demodulation of digital messages.
To take decisions pertaining to the successive values of symbols, a symbol synchronizing signal is established from the demodulated digital signal, it being assumed, besides, that the frame synchronization of the digital messages has been done.
2. Description of the Prior Art
Conventionally, for digital messages comprising a sequence of two opposite levels characteristic of a succession of binary symbols (+1 and -1), the points where the signal reaches zero are used to determine the symbol synchronizing signal. This is done by detecting the points at which the signal crosses zero either in one direction only (the rising direction, for example) when the signal exhibits no d.c. offset or in both directions (rising and descending) so as to establish the mean positions of the transitions and hence, the sampling instants (in the case of d.c. offsets). This method is suitable when the signal passes through zero a sufficient number of times and for two-level messages. By contrast, when the signal passes zero infrequently, there is a risk that the transitions between symbols will not be precisely determined. Furthermore, this method cannot be applied to messages comprising symbols chosen from an alphabet of several symbols associated with more than two levels. In the latter case, the inter-symbol transitions do not more necessarily coincide with the points at which the digital signal passes zero.
Finally, when there is a d.c. level offset (for example at the output of a frequency discriminator), it is no longer possible to position the transitions precisely, thus giving rise to errors when taking decisions on the symbols.